Heart muscle normally grows by cellular replication (hyperplastic growth) throughout fetal life. In the perinatal period, the myocardial growth pattern switches. Myocardial hyperplasia irreversibly ceases, i.e., myocytes become terminally differentiated, and all future myocytic growth is via hypertrophy. The clinical problem with this is that cardiac myocytes do not regrow after myocyte loss. If new heart muscle could be regrown after myocardial damage or loss, the organism would have a tremendous survival advantage. To date, the molecular mechanisms (the "switch") that controls the shift from proliferative to hypertrophic growth are unknown. The applicant proposes to characterize the events that are hypothesized to activate this molecular switch. Recent evidence indicates that the MAP kinase signaling cascade may control hyperplastic and hypertrophic growth in cardiac myocytes. The underlying hypothesis for this work is that a molecular switch changes the MAP kinase cascade from mediating hyperplasia to hypertrophy. Concurrent with or consequential to the activation of this switch, cardiac myocytes undergo terminal differentiation. The working hypothesis is that decreased MAP kinase activity is the critical event that activates this switch and allows cardiac myocyte terminal differentiation to proceed and hypertrophic growth to follow. Three questions will be addressed: 1) Is the withdrawal of MAP kinase activity required for terminal differentiation to proceed? 2) Once cardiac cells have become terminally differentiated, is MAP kinase activation necessary or sufficient to stimulate hypertrophy? 3) What molecular events underlie this switch in MAP kinase activity during development? The specific aims are to: (1) test whether MAP kinase activation is necessary and sufficient to mediate hypertrophy in cultured neonatal rat heart cells; (2) test the hypothesis that persistently activated MAP kinase will delay terminal differentiation in the hearts of transgenic mice. Constitutively activated MEK will be used to increase MAP kinase activity, the cardiac myosin light-chain 2v promoter will provide cardiac-specific targeting, and control of transgene expression will be provided by a tetracycline transactivator. Cardiac growth will be evaluated in transgenic animals at 1, 4, 9, 15, 21, 28 and 60 day old mice with the MEK transgene turned on at the time of birth, and at 7 day intervals until 28 days of age; (3) test the hypothesis that decreased MAP kinase activity, accomplished by over-expressing the MAP kinase phosphatase MKP1, will accelerate terminal differentiation in the heart of a similarly designed murine transgenic model.